A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays and other devices involving fine structures. In a conventional lithographic apparatus, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer or glass plate) that has a layer of radiation-sensitive material (resist). Instead of a mask, the patterning device may include an array of individually controllable elements which serve to generate the circuit pattern. Lithographic methods using such controllable (i.e. programmable) patterning device may be referred to as maskless techniques.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam of radiation in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
During the exposure process the substrate is typically supported on a substrate table, and a projection system is used to project the patterned beam onto a surface (the target surface) of the target portion. The correct positioning of the target surface of the substrate with respect to the projection system is desirable to achieve the desired pattern on the substrate. For example, in maskless techniques, it is typically desirable to arrange the projection system such that it focuses the patterned beam to form a pattern of radiation spots on the target surface, each spot corresponding to a respective one of the controllable elements. The projection system is thus arranged to focus the patterned beam onto a target plane, and the substrate should be mounted on the substrate table such that the target surface is coincident with the focal plane. A problem, therefore, is how to achieve correct support of the substrate; if too high or low the patterned beam may not be correctly focused on the entire target area, and if the substrate is inclined with respect to the focal plane (i.e. if it is not “level”) then only a portion of the target area may be in correct focus.
There is continual motivation for lithographic apparatus and methods to expose larger and larger substrates. One motivation for extending the size of the substrate is to reduce the manufacturing cost per device (chip, die, or display etc.). Normally, the integrated circuit (IC) or flat panel display (FPD) manufacturing technology evolves in terms of generations, and as the substrate sizes become larger and larger, the throughput (wafers per hour) or TACT time (Turn Around Cycle Time for processing one substrate) remains more or less the same. Thus, production efficiency is increased. Using FPD as an example, the glass substrate size (surface area) may increase by a factor of approximately 1.8 in going from one generation to the next, while the TACT time remains more or less constant (60 to 80 seconds). Thus, productivity increases by a factor of approximately 1.8 in going from one generation to the next. In flat panel display (FPD) technology, there is a continual desire to produce larger single screens, each being based on a single substrate over which devices have been produced using lithographic techniques. It will be appreciated that, as the substrates themselves become larger, the above-mentioned problems associated with the correct positioning of target surfaces with respect to projection systems, and those associated with the support of the substrate on rigid, moveable stages, are exacerbated.
Adjusting the focus and leveling for an FPD exposure system is challenging because, firstly, the substrate size may be large. It is envisaged that substrate sizes may be as large as 1.85×2.25 meters in the near future, and may be even larger as time progresses. Secondly, the total thickness variation (TTV) over such substrates is large. A typical TTV of a FPD substrate is 20 microns. Another problem is that the image field (in general terms, the size of the pattern projected onto the target surface in a single exposure step) is large. It is desirable to image single LCD TV screens (and indeed other forms of flat panel TV screens, such as PLED, OLED etc.) without stitching. This means that the exposure field of the lithography system will be at least as big as the largest display to be made (in the case of a stepper) and at least as long as one side of the largest display (in the case of a scan and step system). Currently, LCD TV screens are being produced with diagonal dimensions of about 32″. In the near future LCD TV screens with diagonals in excess of 54″ are anticipated. It will be appreciated that the projection system (optics) for projecting images onto such large LCD screens is itself large. Thus the problem of adjusting the focus and leveling of the exposure system by adjusting the projection optics alone is becoming increasingly difficult.